Implement DatetimeIndex.normalize() and DatetimeIndex.strftime()

[xinrong-meng]

>>> idx = ks.date_range(start='2014-08-01 10:00', freq='H', periods=3)
>>> idx.normalize()
DatetimeIndex(['2014-08-01', '2014-08-01', '2014-08-01'], dtype='datetime64[ns]', freq=None)

>>> idx = ks.date_range(pd.Timestamp("2018-03-10 09:00"), periods=3, freq='s')
>>> idx.strftime('%B %d, %Y, %r')  # doctest: +NORMALIZE_WHITESPACE
Index(['March 10, 2018, 09:00:00 AM', 'March 10, 2018, 09:00:01 AM',
       'March 10, 2018, 09:00:02 AM'],
      dtype='object')

[codecov-io]

Codecov Report

Merging #2089 (282dbc7) into master (2e6094b) will decrease coverage by 3.43%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2089      +/-   ##
==========================================
- Coverage   95.28%   91.84%   -3.44%     
==========================================
  Files          57       57              
  Lines       13223    13115     -108     
==========================================
- Hits        12600    12046     -554     
- Misses        623     1069     +446     

Impacted Files	Coverage Δ
databricks/koalas/missing/indexes.py	100.00% <ø> (ø)
databricks/koalas/indexes/datetimes.py	96.69% <100.00%> (-1.60%)	⬇️
databricks/koalas/tests/indexes/test_datetime.py	96.96% <100.00%> (-3.04%)	⬇️
databricks/koalas/plot/plotly.py	15.78% <0.00%> (-81.06%)	⬇️
...bricks/koalas/tests/plot/test_frame_plot_plotly.py	23.33% <0.00%> (-76.67%)	⬇️
...ricks/koalas/tests/plot/test_series_plot_plotly.py	26.92% <0.00%> (-71.26%)	⬇️
databricks/koalas/typedef/typehints.py	79.88% <0.00%> (-14.53%)	⬇️
databricks/koalas/__init__.py	78.66% <0.00%> (-13.34%)	⬇️
databricks/koalas/accessors.py	86.43% <0.00%> (-7.04%)	⬇️
databricks/conftest.py	94.54% <0.00%> (-5.46%)	⬇️
... and 26 more

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[ueshin]

LGTM.

[ueshin]

Thanks! merging.

[xinrong-meng]

Thank you!